1. Field of the Invention
The present invention relates to a heavy duty pneumatic radial tire, particularly to an improvement of stability in the running operation, particularly in the braking operation of a heavy duty pneumatic radial tire by improving performance of the tire on wet road, particularly resistance against both the lateral sliding due to skid and the slip.
2. Description of the Prior Art
Heavy duty pneumatic radial tires are generally fitted to trucks or buses, and used. When these tires are used, there is a great difference in the wheel load between the laden running, wherein a given amount of load is imposed to a car, and the unladen running, wherein the car is empty, and the ground-contact area in the unladen running is about one-half of the ground-contact area in the laden running.
The performances on wet road of these tires, which are demanded to these tires for the purpose of stable running of vehicles, are highly influenced by the kind of tread patterns. Merits and demerits of commonly known tread patterns are as follows.
Rib type pattern:
Ground-contact area of a tire is very small in the empty car as described above, and therefore the traction of the tire having the pattern of this type in the empty car is poor, but the tire of this type has excellent resistance against lateral sliding even in an empty car in the cornering test. PA1 In the tire having a block type pattern, the traction does not substantially decrease even in an empty car contrary to the tire having a rib type pattern, but in the tire having a block type pattern, the resistance against lateral sliding at the cornering under unloaded condition is considerably lower than that under loaded condition. PA1 Lug type pattern is inferior to the above described rib type pattern and block type pattern in both the lateral sliding resistance at the cornering and the traction at the straight running, particularly in the empty car. PA1 This pattern has merely a compromised property of both the patterns, and can not improve satisfactorily the performances aimed in the present invention.
Block type pattern:
Lug type pattern:
Rib-block composite type pattern, such as side rib-center block:
The inventors have made repeatedly a large number of experiments with respect to the ground-contacting behavior of a heavy duty pneumatic radial tire under loaded and unloaded conditions. As the a result, the inventors have found that, in the heavy duty pneumatic radial tire, the ground-contact area is very small in an empty car, and only the center portion of the tread acts on traction, and the tire is apt to slip at braking and skid to lose the cornering stability of the tire and to cause the lateral sliding of the tire; and that lands located at both side region, particularly the edge portion of the lands, have a high influence upon the lateral sliding even in the above described decreased ground-contact area.
The inventors have further studied based on the above described discovery, in order to overcome the drawbacks of the above described rib type tread pattern by forming a tread pattern that has a high contribution to skid resistance at both side regions of the tread and can obtain a satisfactorily high traction at the center region of the tread even under a local ground-contact at the tread center region to cause hardly slip. As the result, the inventors have proposed a tread pattern having the following arrangement, and attained advantageously the object of the present invention.